Allow multiple players per test

Refactors the test helpers to use receiver functions so multiple players
can exist in the world per test case.  The AddFriendsTest is an example
of where this is needed.

Additionally, Hamcrest has been replaced with AssertJ for fluent
assertions.

game/build.gradle

@@ -33,9 +33,7 @@ dependencies {
     compile group: 'org.jetbrains.kotlin', name: 'kotlin-stdlib-jre8', version: "$kotlinVersion"
     compile group: 'org.jetbrains.kotlin', name: 'kotlin-compiler-embeddable', version: "$kotlinVersion"
 
-    // https://mvnrepository.com/artifact/org.hamcrest/hamcrest-all
-    testCompile group: 'org.hamcrest', name: 'hamcrest-all', version: '1.3'
-    testCompile "org.jetbrains.kotlin:kotlin-test-junit:$kotlinVersion"
+    testCompile group: 'org.assertj', name: 'assertj-core', version: '3.8.0'
 }
 
 task run(type: JavaExec, dependsOn: classes) {